A method of determining the properties of fiber pulp by separating a batch of a predetermined volume, by measuring its temperature, and by filtering water from it by means of a pressure difference. In accordance with the present invention, substantially all the water is filtered off from the pulp mixture, an air flow is produced through the filtered pulp cake by means of a pressure difference, the resisting effect of the pulp cake on the flow of air is measured, and the pulp cake is weighed, whereafter the properties of the pulp are calculated on the basis of the measurements, from mathematical models developed for this purpose.The apparatus developed for carrying out the method according to the invention has a vessel equipped with a stirrer for the specimen, a device for measuring the temperature of the specimen, means for directing the specimen onto the wire and for removing the water through the wire by means of a pressure difference, a device for measuring the pressure difference or through-flow produced across the wire and the pulp layer on the wire, and devices for registering the obtained measurements and for the output of the properties of the pulp. For directing a batch of a predetermined volume from the vessel into the measuring vessel placed against the wire, this determination apparatus has a specific portioning device, and the device has been fitted not to register the pressure difference or throughflow produced across the wire and the pulp layer on the wire, until substantially all the water has been removed from the pulp layer filtered onto the wire. In addition, this apparatus has means for removing the pulp layer from the wire and a device for weighing the pulp layer.